FIG. 5 shows a conventional step-down circuit for power supply voltage. The step-down circuit for power supply voltage is provided with a reference voltage generation circuit 14 for generating a reference voltage V.sub.REF from an external power supply voltage V.sub.CC, a differential amplification circuit 11 to which the reference voltage V.sub.REF and a power supply voltage V.sub.INT for use in an internal circuit 13 are inputted, and a driving circuit 12 for controlling a driving current I.sub.INT for the internal circuit 13 by receiving a control signal V.sub.OPO that is an output of the differential amplification circuit 11. A P-channel MOSFET is employed as the driving circuit 12.
Upon activating the internal circuit 13, as the power consumption of the internal circuit 13 (that is, the driving current I.sub.INT) increases, the power supply voltage V.sub.INT of the internal circuit 13 decreases. In this case, the control signal V.sub.OPO released from the differential amplification circuit 11 goes low, thereby turning on the driving circuit 12 (P-channel MOSFET). As a result, since the driving current I.sub.INT is supplied to the internal circuit 13, the power supply voltage V.sub.INT of the internal circuit 13 increases.
When the power supply voltage V.sub.INT of the internal circuit 13 keeps on increasing and exceeds the reference voltage V.sub.REF, the control signal V.sub.OPO from the differential amplification circuit 11 goes high, thereby turning off the driving circuit 12 (P-channel MOSFET). As a result, since the driving current I.sub.INT is no longer supplied to the internal circuit 13, the power supply voltage V.sub.INT stops increasing at the time when the power supply voltage V.sub.INT becomes equivalent to the reference voltage V.sub.REF.
As described above, the power supply voltage V.sub.INT of the internal circuit 13 is returned to the reference voltage V.sub.REF by controlling the driving circuit 12 for the internal circuit 13 by the use of the control signal V.sub.OPO that is obtained by detecting and amplifying a difference between the power supply voltage V.sub.INT and the reference voltage V.sub.REF by the use of the differential amplification circuit 11. Thus, the power supply voltage V.sub.INT of the internal circuit 13 is set at the reference voltage V.sub.REF that is lower than the external power supply voltage V.sub.CC.
However, in such an arrangement wherein the difference between the power supply voltage V.sub.INT and the reference voltage V.sub.REF is amplified by the differential amplification circuit 11, there has arisen a problem that in the event of a drastic change in the consumption current of the internal circuit 13 as is illustrated in FIG. 6(b) it takes some time (during time t.sub.2 in FIG. 6(a)) for the control signal V.sub.POP from the differential amplification circuit 11 to go low. For this reason, during this period of time the power supply voltage V.sub.INT drops to a substantial degree from the reference voltage V.sub.REF (as indicated by .DELTA.V.sub.2 in FIG. 6(a)). Consequently, this causes an adverse effect on high-speed operation of the internal circuit 13 that is constituted of a device such as a semiconductor integrated circuit.